Messages in this thread | | | Date | Wed, 31 Jan 2018 15:32:23 +0000 | From | Patrick Bellasi <> | Subject | Re: [PATCH v3 2/3] sched/fair: use util_est in LB and WU paths |
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On 25-Jan 20:03, Pavan Kondeti wrote: > On Wed, Jan 24, 2018 at 07:31:38PM +0000, Patrick Bellasi wrote: > > > > > > + /* > > > > + * These are the main cases covered: > > > > + * - if *p is the only task sleeping on this CPU, then: > > > > + * cpu_util (== task_util) > util_est (== 0) > > > > + * and thus we return: > > > > + * cpu_util_wake = (cpu_util - task_util) = 0 > > > > + * > > > > + * - if other tasks are SLEEPING on the same CPU, which is just waking > > > > + * up, then: > > > > + * cpu_util >= task_util > > > > + * cpu_util > util_est (== 0) > > > > + * and thus we discount *p's blocked utilization to return: > > > > + * cpu_util_wake = (cpu_util - task_util) >= 0 > > > > + * > > > > + * - if other tasks are RUNNABLE on that CPU and > > > > + * util_est > cpu_util > > > > + * then we use util_est since it returns a more restrictive > > > > + * estimation of the spare capacity on that CPU, by just considering > > > > + * the expected utilization of tasks already runnable on that CPU. > > > > + */ > > > > + util_est = cpu_rq(cpu)->cfs.util_est_runnable; > > > > + util = max(util, util_est); > > > > + > > > > + return util; > > > > I should instead clamp util before returning it! ;-) > > > > > May be a separate patch to remove the clamping part? > > > > No, I think we should keep cpu_util_wake clamped to not affect the existing > > call sites. I just need to remove it where not needed (done) and add it where > > needed (will do on the next iteration). > > cpu_util_wake() is called only from capacity_spare_wake(). There are no other > callsites.
True...
> The capacity_spare_wake() is clamping the return value of > cpu_util_wake() to CPU capacity.
... actually it's clamping negative numbers with:
max_t(long, capacity_of(cpu) - cpu_util_wake(cpu, p), 0);
thus, by having cpu_util_wake returning potentially a value which is bigger then capacity_of or capacity_orig_of we should not have issues from a capacity_spare_wake() usage standpoint.
> The clamping is not needed, I think.
However, we can still argue that the cpu_util_wake() should never return something bigger then the maximum possible capacity of a CPU. At least that's the feature so fare.
Thus, even just for the sake of consistency, with previous returns paths (e.g. when we bail out returning cpu_util), I would say that it's worth to maintain this semantics.
With a clamping, all these functions: - cpu_util - cpu_util_est - cpu_util_wake will always return a signal which is never bigger then the maximum possible CPU capacity.
-- #include <best/regards.h>
Patrick Bellasi
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